The present relates to imaging array integrated circuits. More particularly, the present invention relates to imaging arrays having improved transmission of light from an upper surface to the detector regions of the integrated circuit.
As digital cameras become thinner, the angles of light irradiating the individual pixel sensors in the imaging array become larger as measured normal to the surface. Designers have employed several techniques to accommodate these angles.
According to one possible solution, the pixel sensors that make up the array can be increased in size at the cost of decreasing resolution. This is generally not considered to be a satisfactory solution in view of the trend to increase rather than decrease the resolution of digital cameras.
In very small pixels, such as those used for cell-phone camera sensors, a “light pipe” has been employed. This is similar in concept to a fiber optic cable, relying upon total internal reflection (TIR). It therefore requires the use of a high-index polymer as the core of the light pipe. The concept will work well for small incident angles (steep angle of incidence on the sidewall), but it becomes progressively less useful as incident angles increase. According to one particular prior-art light-pipe solution depicted in FIG. 1, light pipes employing internal reflection at the edges of lenses are positioned over the pixel sensors. Adjacent pixel sensors 10a and 10b are shown formed in p-type substrate (or well) 12. Dielectric layer 14 is formed over the pixel sensors 10a and 10b and vias are formed, respectively over and in alignment with pixel sensors 10a and 10b and are both filled with a polymer to form light pipes (indicated at reference numerals 16a and 16b) having a high index of refraction (e.g., n≅1.6). Lenses 18a and 18b are formed on the surface of the dielectric layer as is known in the art. A layer of material (shown by reference numerals 20) provides total internal reflection is formed at the edges of the lenses 18a and 18b between adjacent pixel areas.
Light rays directed at the surface of the pixel sensor array containing pixel sensors 10a and 10b, two of which are shown symbolically at reference numerals 22. As shown in FIG. 1, the light rays bend at the interface of the lenses 18a and 18b. The light rays 22 are also shown reflecting from the layer 20 at the edges of the lenses. Without the presence of the layers of material 20, these light rays 22 would continue along a path that would lead into the next adjacent pixel but the presence of the layer of reflective material 20 reflects them back into the pixel area into which they entered.
As the light rays 22 continue downward from the lens into the polymer layers 16a and 16b, they are reflected by the interface (shown at reference numerals 24a and 24b) between the respective polymer layers 16a and 16b and the dielectric layer 14 (having an index of refraction of about n=1.53) in which they are formed. This interface is not 100% reflective and so some of the light shown at reference numerals 26 passes through the interface, through the dielectric layer separating the two adjacent pixels, and undesirably into adjacent pixel sensors causing undesirable crosstalk.
According to another particular prior-art solution depicted in FIG. 2, back-side illumination (BSI) has been used. In this embodiment, which is shown in a vertical orientation opposite to the orientation of FIG. 1, a typical photodiode pixel sensor 10 is shown formed in substrate or well 12. Dielectric layer 14 formed over the pixel sensor 10 includes transistor diffusions 28 and metal interconnect segments 30 both shown at arbitrary locations for purposes of illustration only.
The microlens 18 for the pixel sensor 10 is formed over a silicon dioxide layer 32 on the backside 34 of the silicon wafer on which the pixel sensor is fabricated. The silicon dioxide layer 32 is much thinner than the dielectric layer 14 of the prior-art example shown in FIG. 1, and therefore accepts light from a relatively large angle.
All of these techniques have disadvantages. Ideally, it would be desirable for a small pixel to have the same acceptance angles as a large pixel without the drawbacks of the present solutions.